DESCRIPTION (Investigator's Abstract): While much is known about the visual properties of neurons in the primary visual cortex, the roles that these cells play as members of cortical circuits responsible for image representation and early processing remain largely unknown. Membership in these circuits necessarily involves interactions spanning several time scales and several levels of organization. The work proposed here is designed to elucidate the circuitry and identify the cellular or synaptic sites of contrast gain control mechanisms and the global suppression of responses elicited by stimuli other than those which excite cells. It is hypothesized that the global suppression is one manifestation of a process by which cellular responses are normalized by total local contrast energy and that this process involves a GABAergic feedback which interacts nonlinearly with feedforward excitation mediated by NMDA receptors for glutamate. This feedback mechanism provides an important key to understanding the representation of complex images by ensembles of neurons. In addition, it is hypothesized that the feedforward connections are self-modifying and provide a variable synaptic gain which accounts for observed pattern-specific adaptation of cortical neurons and shifts of their operating points along the contrast axis. In the course of this work, it is proposed that special emphasis be given to a little-studied class of simple cells located in layer IV whose receptive fields resemble those of cortical afferents most closely. The significance of this work lies in the insights it will provide into global attributes of cortical function. The energy normalization process is of particular importance since a host of recent theoretical studies have emphasized the utility of starting from an energy based representation in order to extract information about motion, texture, stereoscopic depth and shape. At present, the biological foundations of these studies is shaky at best.